1. Field of the Invention
This invention, in its preferred form, relates to apparatus for laser machining a work piece and more particularly to apparatus for collecting and removing particles or fines produced in the course of laser machining. More particularly, this invention relates to apparatus for sealing a welding chamber while the laser machining takes place, for directing an inert gas into the welding chamber, for removing the inert gas with the fines suspended therein and for collecting the fines.
2. Description of the Prior Art
The precision laser machining apparatus of this invention relates generally to the manufacture of nuclear fuel bundle assemblies 10 as shown in FIG. 1 of the drawings. As shown, the nuclear fuel bundle assembly 10 is a self-contained unit comprised of a top nozzle assembly 12 and a bottom nozzle assembly 14, between which is disposed a matrix of nuclear fuel rods 18 arrayed in rows and columns and held in such configuration by a plurality of fuel rod grids 16. Though not shown in FIG. 1, control rods are included at selected positions within the array of nuclear fuel rods 18. The assemblies 12 and 14 and the fuel rod grids 16 provide a skeletal frame to support the fuel rods 18 and the control rods. The nuclear fuel bundle assemblies 10 are loaded into predetermined locations within a nuclear reactor and, therefore, the orientation of the fuel rods 18 with respect to each other is rigorously controlled.
The precision laser welding apparatus of this invention is, in one illustrative embodiment thereof, related to the manufacture of fuel rod grids 16 as shown in FIGS. 2A to 2C. The fuel rod grid 16 is of an approximately square configuration, whose periphery is formed by four outer grid straps 22. Each end of an outer grid strap 22 is welded by a corner seam weld 30 to the end of a perpendicularly disposed outer grip strap 22. A plurality of inner grid straps 20 is disposed in rows and columns perpendicular to each other, whereby a plurality of cells are formed to receive the control rods and the nuclear fuel rods 18. The inner grid straps 20 disposed along the rows and columns have complementary slots therein at each of the points 24 of intersection for receiving a perpendicularly disposed inner grid strap 20. An intersect weld 32 is formed at each of the points 24 of intersection, whereby a rigid egg crate structure is formed. Further, each of the inner grid straps 20 includes at each end a pair of tabs 26 of a size and configuration to be tightly received in either a top or bottom row of slots 28 formed in the outer grid straps 22, as shown in FIG. 2A. A slot and tab weld 34 is effected along the top and bottom rows formed by the slots 28 within the outer grid straps 22.
Further, a plurality of guide sleeves 36 is disposed on the sleeve side surface of the fuel rod grid 16 to receive and guide the control rods disposed therein. A series of notch seam welds 40 securely attaches the guide sleeves 36 to corresponding notches 38 formed within the inner grid straps 20. The precision laser welding apparatus of this invention is particularly adapted to perform a series of controlled welding operations whereby each of the welds 30, 32, 34, and 40 is carried out. It is understood that after each such weld, the fuel rod grid 16 is repositioned and/or the focal point of the laser beam changed to effect the particular type of weld desired.
Referring now to FIGS. 2B and 2C, the plurality of resilient fingers 44 is disposed longitudinally of the inner grid straps 20 in a parallel relationship to each other. A pair of spacing fingers 46 is disposed on either side of a corresponding resilient finger 44 and serves along with the resilient finger 44 to provide a resilient grip of the nuclear fuel rods 18 that are disposed within the cell formed by the intersecting inner grid straps 20. A resilient finger 44a is disposed to the right as seen in FIG. 2C in an opposing relationship to the spacing finger 46a, whereby a nuclear fuel rod 18 is resiliently held therebetween.
The fuel rod grid 16 is machined and in particular welded. In order to perform the intersect welds 32, the fuel rod grid 16 is incrementally moved along each of its X and Y axes, stopping at each of a plurality of positions wherein the laser beam is aligned with each of the intersections of the inner grid straps 20. Once positioned, a laser source is energized to emit a laser beam onto the aligned point of intersection to thereby effect an intersect weld 32. Thereafter, the fuel rod grid 16 is moved to the next position and another intersect weld 32 is made. The slot and tab welds 34, as well as the corner seam welds 30, are made by rotating the fuel rod grid 16 about its Y axis so that each of its outer grid straps 22 is presented to the laser beam for welding. In addition, notch seam welds 40 securing the guide sleeves 36 within the notches 38 of the inner grid straps 20 are carried out by rotating the fuel rod grid 16 to a position disposed at an angle of 45.degree. with respect to the laser beam to thereby expose the interface between the guide sleeves 36 and the slots 38 to the laser beam. The laser beam is initially focused to perform the intersect welds 32 as are carried out within a single plane in which the intersect welds lie. In order to make the corner seam welds 30 and the slot and tab welds 34, it is necessary to rotate the fuel rod grid 16 out of the plane of the intersect welds 32, thus requiring the refocusing of the laser beam. In similar fashion, the fuel rod grid 16 is rotated from the plane of the intersect welds 32 to its 45.degree. angle position with respect to the laser beam, thus also requiring a refocusing of the laser beam before precision welding may be carried out.
As described in the copending application entitled "ARGON PURGED WELDING CHAMBER" Ser. No. 414,242, U.S. Pat. No. 4,492,843 a laser welding system 102, as shown in FIGS. 3 and 4, controls the series of welds and, in particular, the intersect welds 32, the slot and tab welds 34, the corner seam welds 30, and the notch seam welds 40 necessary to secure the inner and outer grid straps 20 and 22 together to form the fuel rod grid 16 and to secure the guide sleeves 36 to the grid 16. As will be detailed below, FIG. 4 does disclose features not included in any of the above referenced copending applications. As shown in FIG. 3, the laser system 102 emits a laser beam 169 of controlled energy, successively and precisely positions the grid 16, and controls the supply of a suitable inert gas, e.g., argon, into a pair of welding chambers 108a and 108b, wherein the laser welding of the aforementioned welds is carried out. Each of the work pieces, e.g., the fuel rod grids 16, is successively moved to each of the weld positions by its positioning module 106a or 106b. In particular, one of the pair of welding chambers 108a and 108b is associated with each of the positioning modules 106 for receiving its grid 16 to establish an environment in which the laser welding may be carried out and, in particular, to establish an atmosphere of the inert gas while permitting movement of the grid 16 to effect the series of welds.
A main frame 122 is more fully shown in FIG. 4 for mounting adjustably the laser system 102 in an aligned position with respect to the right and left positioning modules 106a and 106b. Once aligned with the laser system 102, the right and left positioning modules 106a and 106b are fixedly secured with respect to the main frame 122 and therefore with respect to the laser system 102 to ensure that the alignment of the laser beam 169 may be accurately controlled with respect to each of the positioning modules 106a and 106b and therefore with respect to the fuel rod grids 16 carried thereby. The main frame 122 is made up of a top plate 142 and a bottom plate (not shown), each welded to a frame of square tubing. The top plate 142 is machined flat after it has been welded to its frame of square tubings to provide a reference surface for the other system components that are mounted thereon. These other components are bolted or doweled to or with respect to the top plate 142 so that the critical alignments can be maintained.
As shown in FIG. 4, each of the positioning modules 106a and 106b includes a slide table 262. Each slide table 262 is mounted for rectilinear movement by a pair of bearing shafts 278 affixed to its lower surface. Each of the bearing shafts 278 slidably moves upon corresponding ones of a pair of pillow blocks 282, whereby the slide table 262 and its welding chamber 108 may be withdrawn from its positioning module 106. Each welding chamber 108 is mounted upon an X-Y platform 244. Each X-Y platform 244 is mounted upon an X-Y positioning system 288 by which its welding chamber 108 is incrementally moved along X and Y axes under the control of a computer, as explained in the above-referenced copending patent application entitled, "ARGON PURGED WELDING CHAMBER". Each of the positioning modules 106 further includes a top or sealing plate 156 that is disposed in a close spacing with and forms the enclosing top surface of its welding chamber 108.
A kinematic support 140, shown in FIG. 4, positions the laser system 102, as shown in FIG. 3. The laser system 102 includes a source of laser emission in the form of a laser rod 170 and the related optics with respect to the reference surface of the main frame 122 and more specifically with respect to the work pieces in the form of the fuel rod grids 16, as shown in FIG. 4. As shown in FIG. 4, the laser rod 170 is disposed within a laser head housing 166 and is mounted upon an optical tooling plate 168 which has been machined to very close tolerances for flatness. In addition to the laser head housing 166, a movable beam switching mirror 172 and its actuator in the form of a linear electric motor (not shown) and stationary beam diverters in the form of mirrors 174, 176a and 176b are also mounted upon the optical tooling plate 168. As shown in FIG. 3, the beam switching mirror 172 is mounted for rectilinear motion into and out of position to reflect or to transmit the laser beam 178 emitted from the laser rod 170.
The laser system 102, as shown schematically in FIG. 3, may, in one illustrative embodiment of this invention, take the form of that laser system manufactured by Raytheon under their model designation number SS500. The laser system 102 includes the laser rod 170 illustratively taking the form of a Nd:YAG crystal laser and a pair of linear krypton flash lamps 186 disposed in a high efficiency, laser head. The laser head includes a total reflecting mirror 182 and a partial reflecting mirror 184 disposed on either end of the laser 170. An inner cavity shutter 188 is disposed between the laser rod 170 and the total reflecting mirror 182 and is selectively controlled to release a selected number of lasing pulses, whereby the energy imparted to effect laser welding may be precisely controlled in a manner to be explained below. The laser head is modularly constructed to permit all optic elements thereof including the laser 170, the excitation lamps 186, and the mirrors 182 and 184 to be easily and independently replaced. The excitation lamps 186 may be quickly replaced without disturbing the optical alignment. Further, the excitation or flash lamps 186 are water cooled over their entire length, including their end connectors. Lamp triggering provides for parallel pulsing of the excitation lamps 186 by energizing the cavity.
A dump shutter 190 is disposable in a first position to direct the laser beam 169 along a diverted path 196 into a beam absorber 194 during those periods in which the work pieces in the form of the fuel rod grids 16 are being changed within the chambers 108. An actuating mechanism 192 is shown for disposing the shutter 190 from its first beam intercepting position to a second position, wherein the beam 169 is focused by a beam expander lens assembly 198 to a beam directing mechanism comprised of the movable beam switching mirror 172 and the stationary mirror 174. When the switching mirror 172 is disposed to intercept the laser beam 169, it is diverted along path 178a to the mirror 176a to be directed vertically downward. The laser focusing lens assembly 204a intercepts and focuses the laser beam 169 directed along path 178a onto the fuel rod grid 16 within the chamber 108a. The laser focusing lens assembly 204 includes a lens 202 and a lens carrier tube 200 as rectilinearly positioned by the Z-axis laser assembly 222, as shown in FIG. 4. When the switching mirror 172 is slidably disposed by the linear electric motor (not shown) from a position intercepting the laser beam 169, it is directed by the stationary reflective mirror 174 along the path 178b to be vertically directed by mirror 176b towards the welding chamber 108b.
As described in the above referenced copending application entitled, "ARGON PURGED WELDING CHAMBER", each of the welding chambers 108 may be withdrawn from its respective positioning module 106 to permit a fuel rod grid 16 that has been welded to be replaced with a new grid 16. The chamber 108b, as shown in FIGS. 3 and 4, is in a position whereby the new fuel rod grid 16 may be readily be disposed therein and mounted upon a rotatable fixture 242. Thereafter, the slide 262 and its welding chamber 108 is redisposed within the positioning module 106. As shown in FIG. 3, the welding chamber 108 has an upper-most flange 109 that is disposed at a relatively close spacing from the sealing plate 156. In the laser welding system, as described in the above identified copending application entitled, "ARGON PURGED WELDING CHAMBER", an inert gas, e.g., argon, is introduced into the welding chamber 108 through ports (not shown) within the rotatable fixture 242 and through a diffuser plate (not shown) disposed in the bottom of the welding chamber 108. After the welding chamber 108 has been reintroduced within its positioning module 106, the rate of flow of argon into the welding chamber 108 is increased, whereby the entrained air and moisture introduced into the welding chamber 108 during the loading process may be purged through a narrow spacing left between the flange 109 and the lower-most surface of the sealing plate 156. Argon overflows the flange 109 and falls through large openings (not shown) within the main frame 122, which serves as an exhaust plenum, as well as a support base. As shown in FIGS. 5 and 6, a duct blower 329 causes a negative pressure within the main frame 122, whereby the collected argon gas is exhausted via a transition duct 328 and an exhaust conduit 258 to the exterior of the building enclosing the laser welding apparatus 102.
Argon will continue to flow at a relatively high rate until the welding chamber 108 is purged of entrained air and moisture aspirated during the grid loading. A moisture sensor and an oxygen detector (not shown) are incorporated within the welding chamber 108 to continuously monitor the chamber atmosphere. When preset moisture and oxygen levels are reached, laser welding will again be permitted. Argon is continued to be introduced into the welding chamber 108 during the welding process, as described above, but at a reduced rate.
The fuel rod grids, as described above with respect to FIGS. 2A, 2B, and 2C, are illustratively made of zircaloy. The welding of the zircaloy grids 16 generates particulate matter or fines which must be removed. The quantity of particulate matter is relatively low, less than one-half gram per grid 16. However, in the course of manufacturing a number of such grids 16, the accumulation of zircaloy fines can be hazardous. The zircaloy fines are pyrophoric, and can cause unexpected explosions and fires.
The zircaloy fines may collect anywhere that the flow of argon carrying the fines passes through a restricted opening. Major accumulations occur on the flange 109, as shown in FIG. 3, and within the main frame 122, as shown in FIG. 4. The main frame includes cross braces (not shown) which isolate portions of the base and prevent cleaning. In addition, in the laser welding system as described in the above referenced copending patent application entitled, "ARGON PURGED WELDING CHAMBER", the flow of gas passes upwardly from the welding chamber 108 and about the laser focusing lens assembly 204, whereby the zircaloy fines deposit on top of the sealing plate 168. The zircaloy fines deposit wherever the gas stream velocity is low enough for a particle to settle under the influence of gravity.